An air line is a tube that carries a compressed air

Compressed air

Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year....

 supply, e.g. to inflate tyres or power compressed-air tool

Tool

A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...

s. Air line is most commonly used for suppling compressed air to air tools in workshops and in air brake

Air brake (road vehicle)

Air brakes are used in trucks, buses, trailers, and semi-trailers. George Westinghouse first developed air brakes for use in railway service. He patented a safer air brake on March 5, 1872. Originally designed and built for use on railroad train application, air brakes remain the exclusive systems...

systems on larger vehicles. The most common sizes of air tubing are one quarter inch and three eights inch outside diameter tubing although a complete range of tubing in both standard and metric sizes is available.